This invention relates to a process and composition for the production of highly heat-resistant insulating coatings on electrical conductors by coating the conductors with solutions containing catalysts and levelling agents of thermosetting amide- and/or imide-modified ester resins of polyhydric alcohols, polybasic carboxylic acids containing carboxyl groups attached to aromatic rings, optionally in admixture with aliphatic carboxylic acids and, optionally, their anhydrides and/or esters, and compounds containing amino groups, and heating the coated conductors to object temperatures above 200.degree. C.
It is known how to produce polyester resins which are suitable for lacquering electrical conductors when dissolved in organic solvents of the phenol, cresol and/or xylenol type. The electrical conductors are insulated by coating them with a solution of the above-mentioned polyester resins, followed by heating at oven temperatures of about 350.degree. C or higher to harden the polyester resins. Conventional lacquer solution generally contain standard lacquer additives and/or hardening catalysts. It is preferred to use lacquer solutions of polyester resins containing 5-membered imide rings in co-condensed form (British Pat. Nos. 939,377; 1,082,181; 1,067,541; 1,067,542 and 1,127,214; Belgian Pat. No. 663,429; French Pat. No. 1,391,834; East German Pat. No. 30,838; German Offenlegungsschrifts Nos. 1,494,454; 1,494,413; 1,937,310; 1,937,311; 1,966,084; 2,101,990 and 2,137,884).
Solvents of the phenol, cresol and/or xylenol type contain phenolic hydroxyl groups. These solvents are extremely undesirable for physiological reasons and, in some countries can only be used under certain conditions. Accordingly, there is a considerable need to avoid the use of these solvents although they are inexpensive and readily obtainable and effectively dissolve the ester resins in question. It is also known to use other solvents, for example N-methyl pyrrolidone, dimethyl acetamide, dimethyl formamide, dimethyl sulphoxide, N-methyl caprolactam and/or dimethyl sulphone (British Pat. No. 1,082,181). Unfortunately, solvents such as these are also physiologically unacceptable, difficult to obtain and hence expensive and/or inadequately dissolve the ester resins so that it is only possible to obtain low-percentage lacquer solutions. Accordingly, the use of these solvents has never acquired any practical significance.
It is also known that insulating coatings can be produced on electrical conductors by coating the conductors with aqueous solutions containing ester resins with carboxyl groups neutralised with bases, especially amines (German Offenlegungsschrifts Nos. 2,509,048; 2,439,385 and 2,439,386). The disadvantage of this process and of the aqueous solutions used therein is that considerable energy is required for evaporating the water during the stoving process, considerable outside energy has to be delivered to the oven, the high surface tension of the water gives rise to difficulties from the lacquering point of view and the acid number of the ester resins containing carboxyl groups has to be very high, for example above about 50, in order to obtain adequate solubility in water after neutralisation. The result of this is that the proportion of amines used for neutralisation has to be high which gives rise to pollution problems. Stability in storage is unsatisfactory on account of hydrolytic degradation processes.